Understanding the causes of variability in auditory neural responses is critical for effective diagnosis of hearing problems and evaluation of neural integrity early in life. Since the Children's Health Act of 2000 (Public Law 106-310) was adopted, most newborns in the US are screened for hearing loss, resulting in more than 65,000 infants being referred every year for further diagnostic auditory testing. Interpretation of these diagnostic tests in infants relies on a thorough understanding of measures from the peripheral and central auditory system. Current norms are based on cross-sectional studies collected from select groups of infants almost 3 decades ago. Because of dramatic changes in health care standards, increased survival rates of infants born prematurely, and new physiological tests that allow evaluation of different anatomical levels of the auditory system, more accurate descriptions of auditory neural responses and relevant normative data are necessary. The proposed research has three specific aims. In the first aim, we will construct models of auditory brainstem responses over time in a diverse and representative infant sample based on objective, physiological measurements reflecting peripheral and central processing. Relevant variables such as high risk factors, prematurity of birth, hearing screening outcome, neurodevelopmental delay and language skills will be used to explain variability in the physiological measures. Infants will be followed on an individual level, with responses recorded longitudinally on experimental measures. The statistical method of hierarchical linear modeling will be used to describe the physiological test data. The second aim is to recast the results in a way that predicts the likelihood of neurodevelopmental and language delay based on auditory and medical variables. In the third aim, we will model language growth and describe its trajectory in relationship to auditory factors. An expected outcome of this study is that modern, sensitive measures will be different for infants with normal hearing and development, infants with different categories of hearing loss, infants with delayed auditory development and infants with more global, neurodevelopmental delays. The results from this study will improve our scientific and clinical knowledge of various auditory measures and couple the auditory measures to language and developmental outcomes, influencing the fields of audiology, neonatology, pediatrics, speech-language pathology and early intervention. The proposed research will improve the public health by accelerating the identification of behavioral delays early in life as well as supplying parents and providers with improved information earlier so that they can make more efficient and timely rehabilitation decisions.